1. Field of the Invention
The present invention relates to photolithographic projection and in particular to a reticle, an apparatus for monitoring optical system, a method for monitoring optical system, and a method for manufacturing the reticle.
2. Description of the Related Art
Since photolithography patterns formed on a semiconductor wafer are continually becoming increasingly fine and precise, it becomes more important to arrange precisely the wafer in the focal point. Defocus caused by wrong arrangement of the wafer brings defective products. Therefore, methods for arranging the wafer in the focal point correctly have been developed. For example, asymmetrical diffracting gratings are employed. The asymmetrical diffracting gratings have asymmetrical diffracting portions, which generate phase different of light reaching value that is larger than 0 degrees and lower than 180 degrees. Such asymmetrical diffracting gratings generate a positive first order diffracting light and a negative first order diffracting light in different diffracting efficiencies. When the asymmetrical diffracting gratings are exposed with laser light, projection images on the wafer of the asymmetrical diffracting gratings shift perpendicularly to the optical axis if the wafer is shifted in the direction of the optical axis. The shift of the projection images and the shift of the wafer have linear relation. Therefore, once the linear relation is obtained, it is possible to calculate the wafer position from measured position of the projection images. However, existing method requires preparing two reticles, one for the inspection of the wafer position, and another for exposing the device pattern on the wafer. This is because it is difficult to fabricate both the asymmetrical diffracting gratings and the device pattern on single substrate. Therefore, preparing two reticles leads the development cost to high, and exchanging the reticles interrupts continuous production. Further, defocus may be caused while the reticles are exchanged.